Is a guernsey a uniquely Aussie expression meaning a shirt?
Jersey and Guernsey are both islands in the English Channel famed for the hard-wearing water-resistant woolen jumpers/jerseys that the fishwives used to knit for their seagoing menfolk.
These practical outdoor garments, Jersey sweaters/Guernsey jumpers, were popularly used as sportswear in the nineteenth century. So by extension their name is used throughout the English-speaking world for tops in team colors.
For whatever reason, people from normal countries say Jersey. Aussies use Guernsey and it baffles everybody else.